1. Field of the Invention
This invention relates to electrodeposition of chromium on basis metals, and, more particularly, it is concerned with an activation process by which an adherent chromium electrodeposit is formed on ferrous metal substrates using a high energy efficient chromium plating bath.
2. Description of the Prior Art
The use of high efficiency chromium plating baths which contain halides has been hampered by the inability to obtain adequate coating adhesion to certain basis metals. Such baths are disclosed in Mitsui, J7B-33941 (Sept., 1978); Dillenberg U.S. Pat. No. 4,093,522; Perakh et al., U.S. Pat. No. 4,234,396; and Chessin, U.S. Pat. Nos. 4,450,050 and 4,472,249.
In order to obtain an adequate bond on ferrous substrates, as measured by ASTM B 571-79, with conventional chromium plating solutions which comprise a solution of chromic acid and a catalyst such as sulfate, in combination with various fluorides, the usual technique is to reverse or anodically etch the ferrous workpiece in the plating solution, or in a separate chromic acid containing solution, at a predetermined current density and for a predetermined time.
A table which gives suitable lengths of time for such an etching process is found in "Metal Finishing" 80 (5) 65-8 (1982) by C. H. Peger. The use of certain sulfuric acid and hydrofluoric acid etches for specific stainless steels also is suggested in the publication.
Anodic chromic acid treatments for 400 stainless steel alloys and for low and high carbon steels is disclosed in "48th Metal Finishing Guidebook-Directory" 78, 188-202 (1980) by A. Logozzo. Also recommended are cathodic treatments in sulfuric-fluoride solutions for 300 stainless, for nickel alloys and for cast iron.
An alternative procedure in the prior art is the use of a Woods nickel strike for certain special nickel and cobalt-based alloys. (Boeing Aircraft) BAC5709-5.2j(1).
The use of a ferric chloride-hydrochloric acid solution as a clearing agent for the smut produced by anodic sulfuric etches is disclosed at page 137 of "Hard Chromium Plating" Robert Draper Ltd., Teddington, 1964 by J. D. Greenwood.
Brune and McEnally in "Plating" 42, 1127-32 (1955) describe the use of a magnesium sulfate-sulfuric acid anodic etch solution for preparing ferrous parts for plating. Similarly, ASTM Specification B-242-49T suggests the application of an anodic etch using a sulfuric acid solution containing sodium sulfate. ASTM B177-68 describes the use of sulfuric acid or chromic acid as an activator for chromium electroplating on steel for engineering use.
Chessin in U.S. Pat. No. 4,450,050 describes an activation pretreatment for bonding high efficiency chromium electrodeposits on a metal substrate which is characterized by the pre-step of plating the substrate metal with iron or an iron alloy from an iron salt containing bath.
Herrmann, in U.S. Pat. No. 4,416,758, activates metal substrates in an aqueous alkaline cyanide containing solution using current which is periodically reversed, followed by rinsing and chromium plating.
Cast iron substrates present a particular problem with respect to forming adherent chromium deposits. Several references describe pretreatments for cast iron but neither recommends them for use in chromium plating. Canning Handbook on Electroplating (W. Canning, Ltd., Birmingham, England) 1973, p. 338 and 345. Electroplating and Engineering Handbook (Reinhold) 1955, p. 167. The Graham's method is a pickling or immersion process and does not use electrolysis. Canning proposes a solution which contains chromium acid which is disadvantageous because it creates a waste treatment problem.
It has been found that when these procedures are employed with halide containing high energy efficient chromium plating baths, most ferrous metal substrates are not adequately plated because of inadequate adhesion. It can be speculated that the reducing conditions at the cathode at the initiation of deposition cause the halide ion to be reduced to a form which interferes with the molecular bonding of the chromium to the substrate. In any event, the use of high efficiency chromium plating is limited by the problem of inadequate adhesion.
Accordingly, it is an object of this invention to provide an improved process for forming adherent bright, smooth and hard chromium electrodeposits, particularly from high energy efficient chromium baths on metal substrates, particularly on ferrous metals such as cast irons.
Still another object is to provide an activation solution for electrolytically, preferably anodically, etching a ferrous metal substrate in preparation for electrodepositing an adherent bright, smooth and hard chromium metal deposit thereon from a high energy efficient chromium bath, where the degree of adherence of the chromium deposit can be predetermined by the composition and concentration of the solution.